Recently, a MIMO (Multiple-Input Multiple-Output) has drawn attention as the technique for enabling a high-capacity (high-speed) data communication by effectively using a frequency band. The MIMO is the technique to transmit separate data streams from a plurality of antennas of a transmitter by using a plurality of antennas in both of the transmission and the reception, that is to say, by using the transmitter having a plurality of antennas and the receiver having a plurality of antennas, and individually separate a plurality of transmission signals (data streams) mixed on a transmission path from the signal received by each receiving antenna of the receiver by using a transmission path (channel) estimate value, thereby improving a transmission rate without requiring an enlargement of the frequency band.
FIG. 8 illustrates a configuration example of the previous MIMO transmission system. The system illustrated in FIG. 8 corresponds to a system shown in FIG. 1 of the Non-Patent Document 1 to be described later, and is provided with a MIMO transmitter 100 and a MIMO receiver 200; focusing on substantial parts thereof, the MIMO transmitter 100 is provided with a user selector 101, a channel coder/modulator 102, a beam selector 103, a multibeam former 104, a scheduler 105 and a plurality of transmitting antennas 106, and the MIMO receiver 200 is provided with a plurality of receiving antennas 201, a MIMO/SIMO demodulator 202, a channel decoder 203, a transmission beam measure 204 and a transmission beam/stream determiner 205.
Also, in the MIMO transmitter 100, in the user selector 101, under the control of the scheduler 105, one or more user data stream to be transferred is selected from a plurality of series of user data streams and is input to the channel coder/modulator 102, and in the channel coder/modulator 102, under the control of the scheduler 105, a required error correction coding such as a turbo coding is performed with a specified coding ratio, and after that, obtained bit series is mapped to a specified modulation scheme, for example, a symbol having a signal point (signal of the data channel) such as QPSK (Quadrature Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation) and is modulated. Meanwhile, in the channel coder/modulator 102, in addition to the data channel signal, the signal of the pilot channel (pilot symbol) used for channel estimation and the signal of the control channel (control symbol) transmitting the control information may be multiplexed.
The modulated data thus-obtained is input to the beam selector 103, and in the beam selector 103, under the control of the scheduler 105, the beam used for transmitting the modulated data is selected from a plurality of fixed beams (multibeam) formed by the multibeam former 104 by just the number of streams to be transmitted and the modulated data is transmitted from the transmitting antenna 106 by the selected beam.
For example, assuming that the number of transmitting antennas 106 is four and the number of fixed beams capable of being formed by the multibeam former 104 is four at the maximum, when the number of streams to be transmitted is four, all of the four beams are selected, and in a case of two streams, two beams are selected out of four beams, and in a case of one stream, one stream is selected out of four beams.
On the other hand, in the MIMO receiver 200, a wireless signal transmitted from the transmitting antenna 106 of the MIMO transmitter 100 is received by each receiving antenna 201, and MIMO demodulated or SIMO (Single-Input Multi-Output) demodulated by the MIMO/SIMO demodulator 202, and the user data stream is generated. That is to say, in the MIMO/SIMO demodulator 202, the user data streams multiplexed for each of the transmitting antennas 106 are separated by a method of using an inversion matrix of a channel correlation matrix and a method of using an MLD (Maximum Likelihood Detection) algorithm, based on a channel estimate value (channel matrix) obtained by a correlation calculation of the received pilot symbol and the pilot replica, and the demodulated data is generated.
The obtained demodulated data is input to the channel decoder 203, and an error correction decoding such as a turbo decoding is performed in the channel decoder 203, and decoded data of the user stream received by the data channel may be obtained.
Meanwhile, each signal received at the receiving antenna 201 is also input to the transmission beam measure 204, and a CQI (Channel Quality Indicator) value, which is an index of reception quality, is measured based on the received pilot symbol in the transmission beam measure 204, and one or more beam of which reception quality is the best is determined (selected) based on the obtained CQI value in the transmission beam/stream determiner 205. Then, information including the determined number of beams, corresponding CQI value and the beam ID is generated as feedback information to the MIMO transmitter 100 and is transmitted to the MIMO transmitter 100.
The above-described feedback information is finally reported to the scheduler 105 of the MIMO transmitter 100, and thereby, the scheduler 105 controls the user selector 101, the channel coder/modulator 102 and the beam selector 103 so as to transmit the transmission user data stream as described above by the beam of the number of beams (beam ID) determined (selected) in the MIMO receiver 200 (transmission beam/stream determiner 205) and by the coding ratio and modulation scheme depending on the reported CQI value.
Meanwhile, as disclosed in the Patent Document 1 to be described later, in the closed loop type MIMO transmission scheme, which performs pre-coding on the transmitting side, it is also required to send back the information of the channel matrix or the received weight (weighting coefficient of the multibeam) obtained on the receiving side, as the feedback information to the transmitting side.    Patent Document 1: Japanese Patent Application Laid-Open No. 2005-311902    Non-Patent Document 1: 3GPP TSG RAN WGI meeting #43 (R1-051438), “Multi-beam MIMO for EUTRA Downlink”, Fujitsu, November 2005